Symbiosis & Normal Microbiota
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Last revised: Monday, April 17, 2000
Ch. 28 in Prescott et al, Microbiology, 4th Ed.Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!Copyright 2000. Thomas M. Terry
Types of Symbiosis
- Symbiosis = "intimate living together" between different species
- Several possible types, ranging from harm to mutual benefit.
- Not clearly separated in nature; relationship may change from beneficial to harmful as environment changes.
One benefits, one (host) is not obviously affected either positively or negatively
Both benefit from the association
One benefits, the other (host) is (potentially) harmed
Examples of symbiotic relationships
- Symbiotic Nitrogen Fixation
- Mutualistic symbiosis between bacteria (Rhizobium species) and roots of leguminous plants (alfalfa, clover, vetch, peas, beans, etc.) --> root nodules
- Allows growth in nitrogen-poor soils
- Bacteria provide ammonia by nitrogen fixation, create special anaerobic microenvironments in which this can happen. Plants provide nutrients and shelter.
- When bacteria are removed from nodule, cannot carry out nitrogen fixation. Some of needed gene products come from plant genes.
- Note: there are non-symbiotic nitrogen-fixing bacteria, e.g. Azotobacter. Also other types of symbionts, e.g. Frankia that live in Alder roots, create nodules.
- Ruminants & Resident microbes
- Ruminants (R) are herbivorous animals with four-chambered stomach = rumen.
- R eat grasses containing mainly cellulose, but lack enzymes to digest cellulose.
- Bacteria and Protists in rumen produce cellulases, hydrolyze cellulose to sugar which is then fermented.
- Products include:
methane (from methanogens)
organic acids (acetate, propionate, butyrate).
- Acids are adsorbed by R into bloodstream, provide source of energy.
- Methane must be released by belching, ~2 liters/min. Disease "bloat" when cows can't belch.
- Microbial population totally anaerobic, achieves highest density of bacteria (up to 1012 cells/ml).
- Cellulose digestion is slow process. Animals regurgitate rumen contents back to mouth to facilitate breakdown, "chewing cud".
- Syntrophic symbioses
Cross-feeding symbioses: metabolic products of one organism are required by another
- Ex: H2 is produced by a variety of anaerobic fermentations, as a way of getting rid of electrons from NADH. This same H2 is required by anaerobic methanogens in order to carry out anerobic respiration:4H2 + CO2 ---> CH4 + 2 H2O (Methanogens)
- In absence of H2-producing bacteria, methanogens can't grow.
- Q. for further thought: how would you grow methanogens in a laboratory?
- Lichen symbiosis
- Lichens are associations of fungus (host) with photosynthetic alga or cyanobacteria (symbiont).
crustose lichens, commonly found on rocks and trees
- Resulting symbiotic organisms can grow attached to rocks, tree trunks, other unlikely habitats. Fungus (ectosymbiont) provides minerals by releasing lichen acids that dissolve substrate, release small amounts of P, S, other minerals, and obtains water from air. The endosymbiont carries out photosynthesis, converts CO2 to organic matter to feed itself and fungus host.
- Partners in lichen symbiosis can be separated and will grow as individual fungi or alga/cyanobacteria.
- For further information, visit "Lichens" Web page, from "Natural Perspective".
- Gnotobiotic = "known microbiota"; animal host is either entirely free of microbes (aka "germfree", "axenic") or has a microbiota whose identity is completely known.
- Animals in utero are germfree, but acquire resident bacteria within hours of birth.
- Relatively easy to produce germfree animals for birds. Sterilize shell, use sterile incubator, keep animals in an environment where all air, food, water is sterilized before entry.
- More difficult to establish germfree animals other than birds. Need cesarean section of pgrenant females, germfree isolation chambers where all air, food, water is sterilized before entry.
- Germfree animals generally are less healthy than animals with normal microbiota. Defects include:
- Greater vitamin requirements for K and B complex
- lower cardiac output
- much more susceptible to pathogens -- normal microbiota colonize access sites, often compete successfully to prevent pathogens from binding to host tissues.
- much smaller infectious dose required to initiate an infection
Normal Microbiota of Humans
- Internal body organs (heart, kidney, brain, blood) are usually free of microbes. But surface tissues have extensive populations of microbes, called the microbiota, also known as "normal flora" (term that implies some connection with plants, as contrasted with "normal fauna").
- Microbiota are specialists, able to colonize and survive on human tissue. Each body region has characteristic flora; e.g. many streptocococci inhabit nasopharyngeal cavity, while many enteric bacteria inhabit intestinal tract.
- For further information: Bacteriology 330 Lecture Topics: Normal Flora, from U. of Wisconsin.
- Not a great habitat; dries out, constantly being shed, secretions include fatty acids (lower pH to 4-6) and salt.
- Some skin regions better habitats than others: scalp, ears, underarms, anal region are all especially good habitats.
- Bacteria that can grow on skin must be able to survive these conditions.
- Typical bacteria: Propionobacterium acnes. Bacteria can live in sweat glands, hair follicles, so cannot be eliminated by washing skin. P. acnes grows esp. well in skin glands, causes acne when hormone activity in teen years causes overproduction of sebum (fluid secretion).
- Staphylococcus epidermidis, Staph aureus, are found on skin, thrive in nasal region esp.
- Saliva contains lysozyme, other enzymes that kill bacteria.
- But bacteria thrive attached to teeth, esp in gum margins.
- Strep mutans, other streps secrete gooey polysaccharide (---> plaque) that adheres to teeth, provides microhabitat for other bacteria to colonize.
- As food particles accumulate, bacterial growth ----> anaerobic conditions ----> fermentation ----> production of acid wastes ----> tooth decay, gingivitis, periodontal disease.
Bacterial Disease Case Study: Dental caries and Streptococcus mutans
- Teeth in skulls from Europeans prior to the 1500's showed remarkably well-preserved teeth. Once sucrose, a dissacharide from cane sugar, was introduced into the European diet, teeth deteriorated quickly and tooth decay became a widespread disease.
- The bacterium Streptococcus mutans (along with S. sobrinus) play an important role. In the process of breaking sucrose down into its component sugars, glucose and fructose, S. mutans polymerizes all the glucose units into a dextran polysaccharide, using the enzyme dextransucrase.
Light micrograph of Strep. mutans. From Dr. Timothy Paustian, University of Wisconsin-Madison
- Since S. mutans uses lactic acid fermentation exclusively as its catabolic pathway, enormous quantities of dextran are produced when sucrose is present. This accumulates as a gooey polysaccharide matrix which initiates the formation of dental plaque.
- As plaque forms, other bacteria colonize and small food particles are trapped. Rapid bacterial metabolism causes anaerobic environments; lactic and other acids are produced, attacking tooth enamel and causing tooth decay (dental caries).
- Acids also attack surrounding tissues, producing gingivitis (periodontal disease).
- Stomach is highly acidic (pH 2-3), kills most microbes.
- Some bacteria and yeasts can tolerate passage through stomach; few live in stomach.
- Ulcers, long thought to be a "stress disesase", recently shown to be infectious disease, due to Helicobacter pylori. Still not widely known or accepted by medical community.
Bacterial Disease Case Study: Ulcers & Helicobacter pylori
- For centuries ulcers, a disease in which stomach acids attack the lining of the stomach, were thought to be caused by stress and diet. People afflicted with ulcers were taught to change their diet and reduce stress; in extreme cases surgical removal of the stomach was called for.
- About 4 million Americans have ulcers in any given year. See the NIH report on ulcers. For a one-page review, see the NBC page on ulcers.
- In the early 1980's, Drs. Barry Marshall and Robin Warren in Australia identified a bacterium Helicobacter pylori in patients with ulcers. The bacterium survives inside the mucus lining of the stomach by neutralizing stomach acids around itself.
- Marshall and Warren found that antibiotics could be successfully used to treat ulcers.
- H. pylori is found in almost 1/2 of the human population; even though most don't have ulcers, all have some inflammation of the stomach lining (gastritis).
- Read more about the discovery of H. pylori in Jack Brown's "Bugs in the News"
- Small intestins has some bacteria, but digestive enzymes kill. As approach colon, find more and more bacteria, esp. Gram-negative Enterobacteria (e.g. E. coli).
- Colon has enormous bacterial population (1/3 of feces is bacteria). Up to 1012 organisms/gram. Over 300 diff. bacterial species; majority are obligate anaerobes (300x more than facultative anaerobes). E. coli is only 0.1% of total population!
- Bacteria in colon divide every 12-24 hours on average, much slower than laboratory batch culture rates.
- Upper tract (kidney, bladder) usually sterile.
- Lower part of urethra gets some bacteria, but frequently "washed out" by urinary flow.
- When washout is reduced, more chances of urinary tract infections. More common in women because urethra is shorter.
- Vagina has complex microbiota. After women start periods, glycogen is secreted, and lactic acid bacteria produce lactic acid, maintain pH ~ 4.5.
- When this is disrupted (e.g. during antibiotic treatment, or if frequent intercourse leaves lots of semen which raise pH), can allow yeasts to grow = "honeymooner's disease".
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